1. Field of the Invention
The invention relates to a vehicular steering control device and a vehicular steering control method, more particularly, to a steering control device having steering transmission ratio change means and a method for the steering control device.
2. Description of the Related Art
As one vehicular steering control device for an automobile or the like having steering transmission ratio change means, for example, there has been known a steering control device disclosed in the U.S. Pat. No. 6,164,150, which was also filed by the present applicant. This steering control device has an input portion coupled to a steering wheel operated by a driver, an output portion drivingly connected to steered wheels via a steering gear box, and steering transmission ratio change means for changing a steering transmission ratio by causing the input and output portions to rotate relative to each other by means of an actuator that rotationally operates in association with the steering wheel.
In general, the actuator of the steering transmission ratio change means is an electric motor. A main body of the electric motor is supported by the input portion, and a rotational shaft of the electric motor is coupled to the output portion. The electric motor causes the input and output portions to rotate relative to each other, whereby a steering transmission ratio is changed. The main body of the electric motor is supplied with power by an electric connection mechanism from the side of a vehicle body.
As a driver performs a steering operation, the main body of the electric motor rotates relative to the vehicle body, together with the steering wheel and the input portion. The electric connection mechanism has an outer power supply member that is fixed to the vehicle body and that annularly extends around the input portion, an inner power supply member that is fixed to the input portion inside the outer power supply member, and a spiral cable that extends between the outer power supply member and the inner power supply member and that is wound around the inner power supply member a couple of times. The electric motor is supplied with control current through conductor wires contained in the spiral cable. A permissible rotational angle that is formed between the input and output portions and that is restricted by a spiral cable unit is set larger than a relative rotational angle that is formed between the input and output portions and that is determined by a steering mechanism or the like. However, if the actuator is in a certain situation, the permissible rotational angle restricted by the spiral cable unit is insufficient. As a result, an excessive tensile force may be applied to the spiral cable.
For instance, if the steering transmission ratio change means becomes incapable of controlling steering transmission ratio normally owing to a malfunction of a sensor or a high load applied to the actuator, the actuator is stopped and a lock unit is operated. As a result, the input and output portions are prevented from rotating relative to each other. Hence, if the driver rotationally operates the steering wheel greatly in opposite directions after the actuator and the lock unit have been brought out of and into operation respectively due to a malfunction or the like of the sensor under a circumstance where the input and output portions form a relatively large relative rotational angle, the permissible rotational angle restricted by the spiral cable unit can be regarded as having decreased by an angle corresponding to the relative rotational angle formed between the input and output portions. Thus, an excessive tensile force is applied to the spiral cable. This may cause rewinding or breaking of the spiral cable.